Mixing devices are essential parts of environmentally friendly burners for gas turbines. They mix continuous-flow streams of a fuel with an oxidizing fluid, like air, in a burner for premixed combustion in a subsequent combustion chamber. In modern gas turbines good mixing of fuel and oxidizing fluid is a prerequisite for complete combustion with low emissions. Optimization of mixing devices aims at reducing the energy required to obtain a specified degree of homogeneity. In continuous-flow mixing processes the pressure drop over a mixing device is a measure for the energy cost of the mixing procedure. Furthermore, the time and space required to obtain the specified degree of homogeneity are important parameters for evaluating mixing devices or mixing elements.
A high turbine inlet temperature enhances the efficiency in standard gas turbines. As a consequence of high inlet temperatures, there arise high nitric oxide emission levels and higher life cycle costs. These problems can be mitigated with a sequential combustion cycle, wherein the compressor delivers nearly double the pressure ratio of a conventional one. The main flow passes the first combustion chamber (e.g. using a burner of the general type as disclosed in EP 1 257 809 or as in U.S. Pat. No. 4,932,861, also called “EV” combustor, where the EV stands for EnVironmental), wherein a part of the fuel is combusted. After expanding at the high-pressure turbine stage, more fuel is added and combusted (e.g. using a burner of the type as disclosed in U.S. Pat. Nos. 5,431,018 or 5,626,017 or in US 2002/0187448, also called SEV combustor, where the “S” stands for sequential). Both combustors contain premixing burners, as low nitric oxide emissions further require high mixing quality of the fuel and the oxidizer.
Since the second combustor is fed by the expanded exhaust gas of the first combustor, the operating conditions allow self-ignition (spontaneous ignition or auto ignition) of the fuel air mixture without additional energy being supplied to the mixture. To prevent ignition of the fuel air mixture in the mixing region, the residence time therein must not exceed the self-ignition delay time. This criterion ensures flame-free zones inside the burner. This criterion poses, however, challenges in obtaining appropriate distribution of the fuel across the burner exit area. SEV-burners are currently only designed for operation on natural gas and oil. Therefore, the momentum flux of the fuel is adjusted relative to the momentum flux of the main flow so as to penetrate in to the vortices. This is done by using air from the last compressor stage (high-pressure carrier air). The high-pressure carrier air is bypassing the high-pressure turbine. The subsequent mixing of the fuel and the oxidizer at the exit of the mixing zone is just sufficient to allow low nitric oxide emissions (mixing quality) and avoid flashback (residence time), which may be caused by auto ignition of the fuel air mixture in the mixing zone.
Upscaling of the known SEV geometry as currently used for gas turbines like Alstom's GT24 or GT26 to fit can combustors with cylindrical geometry cannot be done without further adaptations since residence time and fluid dynamic structures cannot be maintained at the same time, i.e. known SEV-concepts, as described above, are not working for can combustor.
WO 2011/054760 and WO 2011/054766 describe gas-turbine burners with combined mixing and injection devices for burners with annular or rectangular cross-sections.